1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and an apparatus of performing a cell reselection procedure in a wireless communication system.
2. Related Art
In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and radio data) in addition to the early-stage voice service. Wireless communication systems are designed for the purpose of providing reliable communication to a plurality of users irrespective of users' locations and mobility.
To overcome a restriction on a service area and a limitation in a user capacity, the wireless communication system provides a communication service by dividing the service area into a plurality of cells. This is referred to as a multi-cell environment. A cell selection procedure is a procedure in which a user equipment (UE) selects a cell to receive a service. The purpose of cell selection is to register the UE to a network in order to receive the service from a base station (BS). In general, the UE selects a cell having a strongest signal property value by performing a signal measurement process with respect to the BS in all searchable frequency bands. The cell selected by the UE is referred to as a serving cell. A cell adjacent to the serving cell is referred to as a neighbor cell. A difference between the wireless communication system and a wire communication system lies in that the wireless communication system has to provide a seamless service to the UE with mobility. If the UE moves from one place to another, it may be inappropriate to receive the service from the serving cell. In this case, the UE reselects a suitable cell capable of maintaining quality of service (QoS) and receiving a signal with better quality. This is referred to as a cell reselection procedure. The cell reselection procedure is to select a more suitable cell based on criteria for cell reselection and signal measurement in an idle state. The UE may request the network to provide the service or may wait in the idle state to receive the service from the network. In the idle state, the UE repeats a process of reselecting a cell having a better signal property by measuring signals from the serving cell and the neighbor cell. For the UE with mobility, the cell reselection procedure is important in order to receive a service with better quality.
The neighbor cell may be a radio access technology (RAT) cell different from the serving cell. An RAT is a type of technology used in radio access. Examples of the RAT include a global system for mobile communication (GSM)/general packet radio service (GPRS), a universal mobile telecommunications system (UMTS) terrestrial radio access network (UTRAN), an evolved-UTRAN (E-UTRAN), and so on. In addition thereto, there are various RATs.
The GSM is a radio technology which has been developed as a standardized system for wireless communication systems in Europe. The GPRS was introduced to provide packet switched data services in circuit switched data services provided by the GSM. The GSM/GPRS is a system based on time division multiple access (TDMA). The GSM/GPRS may be referred to as a 2nd generation wireless communication system. The UMTS may be referred to as a 3rd generation wireless communication system based on wideband code division multiple access (WCDMA). The E-UTRAN is a wireless communication system based on orthogonal frequency division multiple access (OFDMA). The E-UTRAN is also referred to as long term evolution (LTE). A GSM cell is a cell using the GSM/GPRS as the RAT. A UTRAN cell is a cell using the UMTS as the RAT. An E-UTRAN cell is a cell using the E-UTRAN as the RAT.
The introduction of various types of RATs results in a problem in the interoperation between the existing GSM/GPRS and a new RAT such as the UMTS. The new RAT preferably maintains backward compatibility with the existing GSM/GPRS in terms of user convenience, and this is also advantageous for a service provider since existing equipment can be reused. For the interoperation between different RATs, the UE may support multi-RAT. The UE supporting the multi-RAT may reselect an RAT cell different from the serving cell. For example, if the serving cell is a GSM/GPRS cell, the UE may reselect a neighbor UMTS cell, E-UTRAN cell, and so on.
Accordingly, there is a need to provide a method and an apparatus of effectively performing a cell reselection procedure if neighbor cells are various types of RAT cells.